Before addressing the present invention, the following background is provided with respect to the development of front and rear suspension systems in modern motorcycles.
Rear swing arm suspensions are now well known in the motorcycle industry and have become common in recent years. In such suspensions the rear wheel of the motorcycle is journaled to the end of a U-shaped fork, or swing arm. The swing arm is hinged to the frame of the motorcycle and extends rearwardly. The rear swing arm and wheel undergo vertical swinging motion in response to road shocks and changing weight loads. The swing arm is spring loaded downwardly, typically by means of a compression spring, so that the swing arm can bear ordinary weight loads as well as road shocks. Various combinations of shock absorbers, dampers, and springs have been used to spring load and damp the swing arm. Road shocks and changing weight loads are typically transmitted via swinging motion of the swing arm to a shock absorber that incorporates a damper as well as the compression spring that provides the necessary downward force on the swing arm. There is commonly provided some mechanism for adjusting the spring force applied to the swing arm in order to adjust the ride of the motorcycle or to accommodate different riders, differing weight loads or changed road conditions.
Front suspension systems of motorcycles have not been as well developed. Until recently the front suspension systems of motorcycles have been virtually universally of the conventional telescoping fork design. Such designs have a recognized disadvantage in that all weight loads and road shocks on the front wheel are transmitted up through the front fork to the frame of the motorcycle, at a point on the frame which is well above and forward of the center of gravity of the motorcycle. This results in the front fork, the attached steering head, and the upper portions of the frame all being necessarily stronger, and thus heavier, than would be necessary to merely bear the torsional loads required to steer the motorcycle. It also results in the entire front suspension system turning with the front wheel, when all that needs to be turned to effect steering is the front wheel.
In U.S. Pat. No. 4,526,249, to Parker, a co-inventor herein, there was disclosed a motorcycle having a novel one-sided front swing arm suspension system. This system includes upper and lower control arms which are hinged to the frame of the motorcycle and which extend forwardly from the frame to a kingpin. The kingpin extends alongside one side of the front wheel at a rake angle which is similar to the rake angle of a conventional telescoping fork, and curves around the outer periphery of the wheel. The front wheel is journaled in a cantilevered fashion to an axle which extends transversely from the lower end of the kingpin. The lower control arm is arcuate and extends alongside of and around the front wheel so as to provide clearance for steering of the front wheel. The lower control arm is connected to the lower end of the kingpin by means of a ball joint which allows the front wheel to be steered while also allowing swinging motion of the control arm. The upper control arm extends forwardly from the frame of the motorcycle and is connected to the upper end of the kingpin, above the front wheel, also by means of a ball joint. The upper and lower control arms swing in parallel so as to absorb road shocks and weight loads on the front wheel. Details of this suspension system are disclosed in the U.S. Pat. No. 4,526,249, which is expressly incorporated herein by reference. The lower control arm of this system will be referred to hereinafter as the front swing arm.
It will be recognized that the advent of the one-sided front swing arm suspension described above creates a motorcycle design having both front and rear swing arm suspension systems. An opportunity afforded by this development is the use of mechanisms for selectively adjusting the amount each of the swing arms are deflected, as a function of the lean angle of the motorcycle.
In this regard, it has been further recognized that one primary advantage of the one-sided front swing arm suspension has been that weight and shock loads on the front wheel can be effectively borne by the motorcycle frame at a point low and close to the center of gravity of the frame, rather than at the point where the steering column is journalled to the frame.